Magnetic bubble memories are well known in the art. Commercially available bubble memories are characterized by patterns of elements, typically Permalloy, responsive to a magnetic drive field reorienting in the plane of bubble movement for moving bubbles along paths defined by those elements. Memories of this type are called "field-access" memories and are organized in the familiar major-minor mode.
A major-minor mode, magnetic bubble memory is characterized by a plurality of closed minor loops and at least one major path or loop. Bubbles recirculate in the minor loops through reference positions from which movement between the minor loops and the major path can occur.
Typically bubble movement between loops occurs by transfer resulting in vacancies remaining in the selected address of the minor loops. The bubble pattern transferred out of the minor loops is advanced to a detector in the major loop and subsequently is returned to the reference positions for transfer back to the selected address. Alternatively, the transferred out pattern is annihilated and substitute data is transferred back into the minor loops. A bubble generator is coupled to the major loop for this purpose. The numbers of stages of the various loops are designed to permit restored or substitute data to be placed in vacancies created in the minor loops by an initial transfer-out operation.
The various operations are controlled by control circuit which includes a counter and an address generator to track numbers of drive field cycles and the instant address of data occupying the reference positions. Transfer occurs in prior art arrangements typically by the application of a current pulse to an electrical conductor electrically coupling the elements defining the above-mentioned reference positions in sequence. But such an electrical conductor is defined in a metallic film overlying the Permalloy pattern and separated from it by an oxide layer. Consequently, significant extra processing is required to provide a conductor driven transfer function. Moreover, a number of transfer points are defined typically. Therefore, the conductor pattern is intricate and there is a reduction in yield associated with it. One problem to which this invention is directed, therefore, is to provide a conductorless transfer. Of course, conductorless bubble transfers are known. U.S. Pat. No. 3,613,058, of P. I. Bonyhard, D. E. Kish, and J. L. Smith issued Oct. 12, 1971, for example, discloses one such arrangement.
Copending application, Ser. No. 209,901 filed on even date herewith, discloses a new arrangement for bubble propagation. The arrangement is of the field-access type involving Permalloy elements and are orienting magnetic field. But the elements are permissive of unexpectedly wide gaps between elements. Specifically, whereas prior art, field-access memories required gaps less than the collapse diameter of a bubble and periods of greater than 4 to 5 times the strip out diameter, the wide gap arrangement permits gaps of about double the collapse diameter. Since, the gap is the limiting feature for bubble memories, the larger the gap, the more dense the memory for any given photolithographic capability.
The particular problem to which this invention is directed is to provide a conductorless transfer for a wide gap, field access bubble memory of the type described in the above-mentioned copending application.